Batteries mentioned hereinafter are all chargeable batteries.
During a first conventional process for charging a battery, curves showing changes of capacity, voltage and current of the battery with time are shown in FIG. 1. First, constant current charging is performed until the voltage of the battery reaches a predetermined value, generally 4.20V. Then, after the voltage of the battery reaches the predetermined value 4.20V, the constant current charging is switched to constant voltage charging in which the predetermined voltage 4.20V is applied, so as to avoid battery performance reducing caused by overcharge of the battery. After the constant voltage charging starts, detection of charging current also starts. When the charging current decreases to 0.001CA (C is the value of the capacity of the battery and A is a measurement unit of current intensity), the charging is terminated. 4.20V is referred to as a charging limited voltage.
The first conventional process has the following disadvantages:
After the constant voltage charging starts, it takes a long time for the charging current to decrease to 0.001 CA from the constant current. Moreover, the closer to 0.001 CA the charging current becomes, the smaller the percentage of the capacity charged into the battery to the whole capacity of the battery becomes. Therefore, the process of the constant voltage charging has a quite low efficiency. In addition, the time when the charging is terminated depends on the capacity of the battery and different batteries may have different capacities. As a result, the current when the charging is terminated is different with respect to different batteries. If a battery whose capacity is unknown is used, the battery may not be fully-charged or it will take a longer time for terminating the charging.
During a second conventional process for charging a battery, curves showing changes of battery voltage and charging current with time are shown in FIG. 2. As shown in FIG. 2, the charging process is described as follows. First, constant current charging is performed until the voltage of the battery reaches a predetermined value VRC. Then, after the voltage of the battery reaches the predetermined value VRC, pulse current is applied to continue the charging. The value of the pulse current is equal to the value of the constant current. During an off period of the pulse current, the voltage of the battery is detected. If the voltage of the battery decreases to a predetermined voltage VRP(VRC>VRP), the pulse current is applied to continue the charging, and a time interval (P) between this pulse current and the previous pulse current is recorded. If the time interval (P) is smaller than predetermined time (Pc), the pulse current is applied to continue the charging. If the time interval (P) is larger than or equal to the predetermined time (Pc), it is determined that the battery has been fully-charged and the charging process is terminated.
The second conventional process has the following disadvantages. If VRC≧charging limited voltage 4.20V, the voltage of the battery after the pulse current charging starts will be higher than 4.2V. As a result, the battery is overcharged and the performance of the batter is reduced. If VRC<4.20V, the pulse current charging starts too early, and thus the charging time becomes longer.
During a third conventional process for charging a battery, curves showing changes of battery voltage and charging current with time are shown in FIG. 3. As shown in FIG. 3, the charging process includes: Step S1: perform constant current charging until the voltage of the battery reaches a predetermined value V1. Because the charging time T0 is long, it is indicated in a clipped manner in FIG. 3. Step S2: after the voltage of the battery reaches the predetermined value V1, switch to apply the pulse current to continue charging the battery. Step S3: stop charging after the battery is charged for a first period of time T1. Step S4: measure passing time An after the charging stops and the voltage of the battery Bn during the passing time An for multiple times, and compare Bn with a second predetermined voltage V2. Step S5: among all values Bn which are higher than V2 when An reaches a second predetermined time T2, deduce, based on data array (An, Bn) of the passing time An and the voltage of the battery Bn, whether the voltage of the battery at a third predetermined time T3 (T3 is larger than T2) will decrease and become smaller than V2 due to stopping charging. Step S6: if it is deduced that the voltage of the battery will decrease to be smaller than or equal to V2, continue the pulse current charging; if it is deduced that the voltage of the battery will not decrease to be smaller than or equal to V2, stop charging. Vertical coordinates in FIG. 3 are just used for describing relative values of V1 and V2, and therefore the values of the voltage are also represented in the clipped manner.
The third conventional process has the following disadvantages. If V1≧charging limited voltage 4.20V, the voltage of the battery after the pulse current charging starts will be higher than 4.2V. As a result, the battery will be overcharged and the performance of the batter will be reduced. If V1<4.20V, the pulse current charging will start too early, and thus the charging time becomes longer.